JP2004117884A

JP2004117884A - Image forming apparatus

Info

Publication number: JP2004117884A
Application number: JP2002281548A
Authority: JP
Inventors: Kenichi Iida; 飯田　健一; Yasuo Yoda; 依田　寧雄
Original assignee: Canon Inc
Current assignee: Canon Inc
Priority date: 2002-09-26
Filing date: 2002-09-26
Publication date: 2004-04-15

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus by which the occurrence of the density unevenness of an image due to the secondary transfer pre-electrification of a developer image from an intermediate transfer body to a transfer material is prevented in the image forming apparatus possessing the intermediate transfer body to which the developer image is primarily transferred from an image carrier where the developer image is formed on a surface and secondarily transferring the developer image primarily transferred to the surface of the intermediate transfer body to the transfer material. <P>SOLUTION: The image forming apparatus having a secondary transfer pre-electrifying means 14 to electrify the developer image on the surface of the intermediate transfer body 8 before secondary transfer so as to have the same polarity and a member 11 opposed to the secondary transfer pre-electrifying means possesses a destaticizing means 15 to destaticize the intermediate transfer body 8 at the right downstream part of the member 11 opposed to the secondary transfer pre-electrifying means in the moving direction of the intermediate transfer body 8, and the surface resistivity of the contact part of the member 11 opposed to the secondary transfer pre-electrifying means with at least the intermediate transfer body 8 is set ≤10<SP>10</SP>Ω/square, and the surface resistivity of the back face of the developer carrying surface of the intermediate transfer body 8 is set in the range of ≥10<SP>8</SP>Ω/square and ≤10<SP>14</SP>Ω/square. <P>COPYRIGHT: (C)2004,JPO

Description

【０００１】
【発明の属する技術分野】
本発明は、電子写真方式や静電記録方式等によって画像形成を行う複写機、プリンタ、ファックシミリ等の画像形成装置に関する。
【０００２】
【従来の技術】
電子写真方式の複数色又はフルカラーの画像形成装置として、各色毎に応じて像担持体としてドラム状の感光体（感光ドラム）を複数配置し、各感光ドラム上に形成された各色の現像剤像（トナー像）を中間転写体、特にベルト状である中間転写ベルト上に順次重ね合わせてカラー画像を形成する画像形成装置、いわゆる、インライン型・中間転写方式の画像形成装置が実用化されている。
【０００３】
こうした中間転写方式の画像形成装置は、例えば特許文献１に記載されているような転写装置、即ち、転写ドラム上に、画像が形成される転写体を張り付けたり、又は吸着したりして、そこへ感光ドラムのような潜像担持体から画像を転写するカラー電子写真装置と比較すると、上記中間転写体を用いたカラー電子写真装置は、転写材になんら加工、制御（例えばグリッパーに把持する、吸着する、曲率をもたせる等）を必要とせずに中間転写体から画像を転写することができるため、封筒、ハガキ、ラベル紙等、薄い紙（４０ｇ／ｍ^２紙）から厚い紙（２００ｇ／ｍ^２紙）まで、幅の広狭、長さの長短、あるいは厚さの厚薄によらず、転写材を多種多様に選択することができるという利点を有している。
【０００４】
又、中間転写体をベルト状とすることで、中間転写ドラムのような剛体のシリンダーを用いる場合と比較して、画像形成装置内部に配置する際の自由度が増して、スペースの有効利用による装置本体の小型化やコストダウンを行うことが出来るメリットもある。
【０００５】
上記した従来のインライン式カラー画像形成装置では、中間転写ベルトは各感光ドラムにそれぞれの一次転写部で接触又は近接しており、各感光ドラム上に形成されたトナー像は、中間転写ベルト上に順次重ね合わせられて転写（一次転写）されることで、中間転写ベルト上に、例えばイエロー、マゼンタ、シアン、ブラックの４色のカラートナー像が形成される。中間転写ベルト上に形成されたカラートナー像は、中間転写ベルトに担持されて、転写材が搬送されて中間転写ベルトに接触する二次転写部において、中間転写ベルト上から転写材に転写され、その後未定着トナー像が転写された転写材は定着装置に到達し、定着装置で加熱・加圧されて永久定着像が得られる。
【０００６】
ここで、中間転写ベルト移動方向で、一次転写部より下流で且つ二次転写部より上流の位置にて、トナー像をそれと同極性に帯電する二次転写前帯電手段が中間転写ベルト表面に対向して設置されているので、中間転写ベルト上に形成されたカラートナー像は、用紙等の転写材への二次転写に先立って、この二次転写前帯電手段からトナー像を構成する現像剤（トナー）と同極性の電荷付与を受ける。
【０００７】
この二次転写前帯電手段による中間転写ベルト表面のトナーの電荷付与により、各色トナートリボの均一化とトリボ値の増加がなされ、この後に続く二次転写効率の向上が図られる。
【０００８】
尚、ここでの二次転写効率とは、二次転写工程において中間転写ベルト上のトナーが転写材上に転写される効率のことである。
【０００９】
【特許文献１】
特開昭６３−３０１９６０号公報
【特許文献２】
特開２０００−２８４６１号公報
【００１０】
【発明が解決しようとする課題】
しかしながら、上述した中間転写ベルトを有する電子写真方式の画像形成装置においては、二次転写前帯電手段による二次転写前帯電により二次転写効率の向上が達成される一方で、この二次転写前帯電に起因する画像の濃度ムラが発生しやすいという問題があった。
【００１１】
この二次転写前帯電に起因する画像ムラとは、二次転写前帯電工程の直後に、中間転写ベルトと、二次転写前帯電手段の対向電極として用いられる二次転写前帯電手段対向部材と、の間で剥離放電が生じることに起因し、この剥離放電の影響により中間転写ベルト表面上のトナーが面内を移動することで、画像の濃度ムラが生じると考えられる。
【００１２】
従って、本発明の目的は、表面に現像剤像が形成される像担持体より現像剤像が一次転写される中間転写体を有し、一次転写された中間転写体表面の現像剤像を転写材に二次転写する画像形成装置において、現像剤像の中間転写体から転写材への二次転写前帯電に起因する画像の濃度ムラの発生を防止した画像形成装置を提供することである。
【００１３】
【課題を解決するための手段】
上記目的は本発明に係る画像形成装置にて達成される。要約すれば、本発明は、表面に現像剤像が形成される像担持体と、一次転写部にて前記像担持体から前記現像剤像が一次転写される中間転写体と、を有し、前記中間転写体上に一次転写された前記現像剤像を二次転写部にて転写材に二次転写して画像を得る画像形成装置であって、
更に、前記中間転写体の移動方向に対して、前記一次転写部の下流で且つ前記二次転写部の上流の位置且つ前記中間転写体の現像剤像担持面側に設置され、前記中間転写体表面に担持された前記現像剤像を、前記現像剤像の帯電極性と同極性に帯電する二次転写前帯電手段と、前記中間転写体を挟んで前記二次転写前帯電手段と対向する位置に設置され、前記中間転写体の前記現像剤担持面の裏面に接触又は近接する二次転写前帯電手段対向部材と、を有する画像形成装置において、
更に、前記中間転写体の移動方向に対して前記二次転写前帯電手段対向部材の直下流部であり且つ前記中間転写体の前記現像剤像担持面の裏面側に設置され、前記中間転写体を除電する除電手段を有し、
前記二次転写前帯電手段対向部材の少なくとも前記中間転写体との接触部分の表面抵抗率が、１０^１０Ω／□以下であり、
前記中間転写体の前記現像剤担持面の裏面の表面抵抗率が、１０^８Ω／□以上、１０^１４Ω／□以下の範囲である、
ことを特徴とする画像形成装置を提供する。
【００１４】
【発明の実施の形態】
以下、本発明に係る画像形成装置を図面に則して更に詳しく説明する。
【００１５】
実施例１
図１は，本発明の実施例１に係る画像形成装置である電子写真方式のプリンタ等のフルカラー画像形成装置を示す概略構成図である。
【００１６】
この画像形成装置は、イエロー色の画像を形成する画像形成部１Ｙと、マゼンタ色の画像を形成する画像形成部１Ｍと、シアン色の画像を形成する画像形成部１Ｃと、ブラック色の画像を形成する画像形成部１Ｋの４つの画像形成部（画像形成ユニット）を備えており、これらの４つの画像形成部は一定の間隔をおいて一列に配置されている。
【００１７】
各画像形成部１Ｙ、１Ｍ、１Ｃ、１Ｋには、それぞれ像担持体としての感光ドラム２ａ、２ｂ、２ｃ、２ｄが設置されている。各感光ドラム２ａ、２ｂ、２ｃ、２ｄの周囲には、帯電ローラ３ａ、３ｂ、３ｃ、３ｄ、現像装置４ａ、４ｂ、４ｃ、４ｄ、転写ローラ５ａ、５ｂ、５ｃ、５ｄ、ドラムクリーニング装置６ａ、６ｂ、６ｃ、６ｄがそれぞれ設置されており、帯電ローラ３ａ、３ｂ、３ｃ、３ｄと現像装置４ａ、４ｂ、４ｃ、４ｄ間の上方には露光装置７ａ、７ｂ、７ｃ、７ｄがそれぞれ設置されている。
【００１８】
感光ドラム２ａ〜２ｄは、本実施例では負帯電の有機感光ドラムで外径３０．０ｍｍであり、アルミニウム等のドラム基体上にＯＰＣ感光層を有しており、駆動装置（不図示）によって矢印方向（反時計方向）にそれぞれ所定の周速度（プロセススピード）で回転駆動される。
【００１９】
接触帯電手段としての帯電ローラ３ａ〜３ｄは、それぞれ感光ドラム２ａ〜２ｄに所定の圧接力で接触し、帯電バイアス電源（不図示）から印加される帯電バイアスによって各感光ドラム２ａ〜２ｄ表面を負極性の所定電位に均一に帯電する。
【００２０】
現像装置４ａ〜４ｄは、本実施例では、二成分現像方式であり、現像バイアス電源（不図示）から印加される現像バイアスによって各感光ドラム２ａ〜２ｄ上に形成される静電潜像に現像剤（トナー）を付着させて、現像剤像（トナー像）として反転現像する。各現像装置４ａ、４ｂ、４ｃ、４ｄには、現像剤として、それぞれイエロートナー、シアントナー、マゼンタトナー、ブラックトナーが収納されている。
【００２１】
接触転写手段としての一次転写ローラ５ａ〜５ｄは，中間転写体としての中間転写ベルト８を介して各感光ドラム２ａ〜２ｄ表面に所定の押圧力で接触し、各転写バイアス電源９ａ、９ｂ、９ｃ、９ｄから印加される前記トナーとは逆極性の転写バイアスにより、感光ドラム２ａ〜２ｄと転写ローラ５ａ〜５ｄとのニップ部である各一次転写部Ｔａ、Ｔｂ、Ｔｃ、Ｔｄで、各感光ドラム２ａ〜２ｄ表面の各色のトナー像を、移動する中間転写ベルト８上に順次重ね合わせて転写（一次転写）する。
【００２２】
露光装置７ａ〜７ｄは、ホストコンピュータ（不図示）からそれぞれ入力される画像情報の時系列電気デジタル画素信号に対応して変調されたレーザ光で各感光ドラム２ａ〜２ｄ表面を画像露光することにより、各帯電ローラ３ａ〜３ｄで帯電された各感光ドラム２ａ〜２ｄ表面に画像情報に応じた静電潜像を形成する。
【００２３】
本実施例の構成において、中間転写ベルト８は、駆動ローラ１１と二次転写対向ローラ１２と従動ローラ１３の３本のローラ部材によって張架されており、駆動ローラ１１の回転駆動によって、感光ドラム２ａ〜２ｄの回転に同期して矢印方向に移動（回転）される。又、駆動ローラ１１の直下流部には、ブラシ状部材である除電部材（除電ブラシ）１５が配置されている。
【００２４】
本発明の特徴である中間転写ベルト８、駆動ローラ１１、除電ブラシ１５の詳細については後述する。
【００２５】
中間転写ベルト８の移動方向に対して、最下流の感光ドラム２ｄと接触する一次転写部Ｔａ〜Ｔｄの下流側で、且つ二次転写対向ローラ１２と二次転写ローラ１９との間に形成される二次転写部Ｔｎ２の上流側に位置する駆動ローラ１１近傍には、二次転写前帯電手段１４が設置されている。又、二次転写対向ローラ１２と従動ローラ１３との間の二次転写部Ｔｎ２の下流側である中間転写ベルト８の表面近傍には、ベルトクリーニング装置１６が設置されている。
【００２６】
次に、上記した本実施例の画像形成装置による画像形成動作について説明する。
【００２７】
画像形成動作開始信号が発せられると、所定のプロセススピード（本実施例では、１１７ｍｍ／ｓｅｃ）で回転駆動する、各画像形成部１Ｙ〜１Ｋの各感光ドラム２ａ〜２ｄは、それぞれ帯電ローラ３ａ〜３ｄによって、一様に本実施例では負極性である所定電位（本実施例では、約−６５０Ｖ）に帯電される。
【００２８】
そして、露光装置７ａ〜７ｄは、ホストコンピュータ（不図示）から入力されるカラー色分解された画像信号を光信号にそれぞれ変換し、変換された光信号であるレーザ光を、各ミラー１０ａ、１０ｂ、１０ｃ、１０ｄを介して帯電された各感光ドラム２ａ〜２ｄ上にそれぞれ走査露光して静電潜像を形成する。
【００２９】
そして、先ず現像装置４ａによって、感光ドラム２ａ上に形成された静電潜像に、感光ドラム２ａの帯電極性（負極性）と同極性の現像バイアスを印加して現像装置４ａに収容されたイエローのトナーを付着させて反転現像を行い、トナー像として可視像化する。
【００３０】
本実施例における現像バイアスは、ＤＣ成分；−４００Ｖ、ＡＣ成分；１．５ｋＶｐｐ、周波数；３ｋＨｚ、波形；矩形波である、直流電圧に交流電圧を重畳したバイアスとした。
【００３１】
そして、このイエローのトナー像は、一次転写部Ｔａにて転写バイアス電源９ａから一次転写バイアス（本実施例では、＋１５μＡの定電流制御）が印加された一次転写ローラ５ａにより、移動（回転）している中間転写ベルト８上に一次転写される。
【００３２】
イエローのトナー像が転写された中間転写ベルト８は、画像形成部位１Ｙの位置部分が駆動ローラ１１の駆動によって画像形成部１Ｍ側に移動する。そして、画像形成部１Ｍにおいても、同様にして感光ドラム２ｂ上に形成されたマゼンタのトナー像が、一次転写部Ｔｂにて一次転写バイアス電源９ｂから一次転写バイアスが印加された一次転写ローラ５ｂにより、中間転写ベルト８上のイエローのトナー像上に重ね合わせて転写される。
【００３３】
以下、同様にして中間転写ベルト８上に重畳転写されたイエロー、マゼンタのトナー像上に、画像形成部１Ｃ、１Ｋの感光ドラム２ｃ、２ｄにて形成されたシアン、ブラックのトナー像を、順に各一次転写部Ｔｃ、Ｔｄにて各転写バイアス電源９ｃ、９ｄから一次転写バイアスが印加された一次転写ローラ５ｃ、５ｄにより、順次重ね合わせて転写してフルカラーのトナー像を中間転写ベルト８上に形成する。
【００３４】
そして、中間転写ベルト８上に形成されたフルカラーのトナー像は、二次転写に先立って、本実施例ではコロナ帯電器である二次転写前帯電手段１４からトナーと同極性（負極性）の電荷付与を受ける。二次転写前帯電手段１４は、駆動ローラ１１に中間転写ベルト８を挟んで対向して設置されて、二次転写前帯電バイアス源１７からバイアス（本実施例では、ＤＣ：−２．５ｋＶ、ＡＣ：４．０ｋＶｐｐ）の供給を受け、中間転写ベルト８表面に形成されたトナー像をそれと同極性（負極性）に帯電する。
【００３５】
本実施例のコロナ帯電器構成に於いては、二次転写前帯電バイアス源１７から供給される総電流（ＲＭＳ値で−１１０μＡ）のうちのおよそ１／５が、二次転写前帯電手段１４の対向部材である駆動ローラ１１に流れる。
【００３６】
この二次転写前帯電により中間転写ベルト８上のトナートリボが、この帯電前の−２０μＣ／ｇ程度から帯電後に−３５μＣ／ｇ程度に上昇する。
【００３７】
そして、中間転写ベルト８上のフルカラーのトナー像先端が二次転写ローラ１９と二次転写対向ローラ１２との間の二次転写部Ｔｎ２に移動するタイミングに合わせて、搬送された用紙などの転写材Ｐをこの二次転写部Ｔｎ２に搬送して、二次転写バイアス電源２０からトナーと逆極性の二次転写バイアス（本実施例では、＋１．７ｋＶ）が印加された二次転写ローラ１９により、転写材Ｐ上にフルカラーのトナー像が一括して二次転写される。
【００３８】
そして、フルカラーのトナー像が形成された転写材Ｐは定着装置２１に搬送され、定着装置２１の定着ローラ２１ａと加圧ローラ２１ｂ間の定着ニップ部でフルカラーのトナー像を加熱及び加圧して転写材Ｐ表面に熱定着した後に外部に排出して、一連の画像形成動作を終了する。
【００３９】
尚、上記した一次転写工程において、感光ドラム２ａ〜２ｄ上に残留している転写残トナーは、ドラムクリーニング装置６ａ〜６ｄによってそれぞれ除去されて回収される。又、二次転写後に中間転写ベルト８表面に残った残トナーは、ベルトクリーニング装置１６によって除去されて回収される。クリーニングされた中間転写ベルト８は、次の画像形成に備える。
【００４０】
上記に説明したように本構成では、中間転写ベルト８上に一次転写されて形成されたフルカラートナー像は、二次転写に先立って、二次転写前帯電手段１４としてのコロナ帯電器からトナーと同極性（負極性）の電荷付与を受け、中間転写ベルト８上のトナートリボを、−２０μＣ／ｇ程度から−３５μＣ／ｇ程度に上昇させる。よって、二次転写効率を上げることができるが、こうした画像形成装置においては、従来例で説明したように、二次転写前帯電手段１４の対向電極として用いられる、ここでは駆動ローラ１１である二次転写前帯電手段対向部材と、の間で剥離放電が生じ、画像ムラの原因となる。
【００４１】
ここでは、二次転写前帯電手段１４は、駆動ローラ１１に中間転写ベルト８を挟んで対向して設置されているので、二次転写前帯電手段対向部材とは、駆動ローラ１１のことである。そして、二次転写前帯電バイアス源１７から供給される総電流のうちのおよそ１／５が、駆動ローラ１１に流れる。もちろん二次転写前帯電手段対向部材が中間転写ベルトを駆動させるローラでなくてもよいことはいうまでもなく、これら中間転写ベルト８を張架するローラ部材とは別に独立して中間転写ベルト８内側に設けられたものでもよい。
【００４２】
そこで、本実施例では、中間転写ベルト８のトナー像担持面の裏面に接触又は近接し、中間転写ベルト８を挟んでトナー像の帯電極性と同極性に帯電する二次転写前帯電手段１４と対向する二次転写前帯電手段対向部材である駆動ローラ１１と、中間転写ベルト８の移動方向に対して、駆動ローラ１１の直下流部で且つ中間転写ベルト８のトナー像担持面の裏面側に、中間転写ベルト８を除電する除電手段、本実施例では除電ブラシ１５を備えている。
【００４３】
除電ブラシ１５によって、中間転写ベルト８と駆動ローラ１１との間で剥離放電を除電し、その発生を抑制しようとするものである。
【００４４】
尚、除電ブラシ１５等の除電部材は、できるだけ駆動ローラ１１のような二次転写前帯電手段対向部材の近くに存在することが望ましく、二次転写前帯電手段１４と対向する位置にあることが望ましい。本実施例では、除電ブラシ１５が、図４に示されるように、中間転写ベルト８と駆動ローラ１１との接点部分から、中間転写ベルト８移動方法下流側に、駆動ローラ１１の半径分の距離より近くの距離にて、駆動ローラ１１と中間転写ベルト８との間の空隙部αである、図では斜線で示される領域α内に除電ブラシ１５の先端が存在するように配置されている。
【００４５】
ここで用いられる中間転写ベルト８としては、ウレタン系樹脂、フッ素系樹脂、ナイロン系樹脂、ポリイミド樹脂などの樹脂や、これらにカーボンや導電粉体を分散させ抵抗調整を行ったもの等を用いることができる。又、特許文献２に記載されるように、帯電緩和時間の短い、いわゆる自己減衰型の電気的特性を有する中間転写ベルトを使用することで、一次転写、二次転写バイアスの制御が簡略化され、良好なフルカラー画像が安定に得られるようになる。
【００４６】
そして、本実施例では、中間転写ベルト８として、ポリイミドにカーボンを分散して表側と裏側における表面抵抗率が共に１０^１０Ω／□となるよう調整し、又、帯電緩和時間の短い、いわゆる自己減衰型の電気的特性を有する、周長１０００ｍｍ、厚み１００μｍの単層の無端ベルトを用いた。
【００４７】
上記表面抵抗率の測定は、ＪＩＳ−Ｋ６９１１に準拠し、導電性ゴムを電極とすることで電極とベルト表面の良好な接触性を得た上で、抵抗計（アドバンテスト社製；Ｒ８３４０）を用いて測定した。測定条件は、印加電圧；１００Ｖ、印加時間；１０ｓｅｃとした。
【００４８】
中間転写ベルト８は、中間転写ベルト８に内包される駆動ローラ１１と二次転写対向ローラ１２と従動ローラ１３の３本のローラ部材によって張架されており、駆動ローラ１１、二次転写対向ローラ１２、従動ローラ１３は電気的に接地されている。尚、ローラ部材の数や種類はこれに限るものではない。
【００４９】
そして、二次転写前帯電手段対向部材となる駆動ローラ１１は、図２に示すように、直径２４．０ｍｍのアルミニウム製の芯金１１ａと、厚さ２．９ｍｍのヒドリンゴム層１１ｂとで構成される外径２９．８ｍｍのローラであり、ヒドリンゴムを抵抗調整することで表面抵抗率を１０^９Ω／□としている。又、駆動ローラ１１の硬度は、ＪＩＳ−Ａ規格で４０°である。
【００５０】
上記表面抵抗率の測定は、ＪＩＳ−Ｋ６９１１に準拠し、ローラ表面に金属製プローブ電極を押し当てることで電極とローラ表面の良好な接触を得た上で、抵抗率計（三菱油化社製；Ｈｉｒｅｓｔａ　ＩＰ　ＭＣＰ−ＨＴ２５０）を用いて測定した。測定条件は、印加電圧；１０Ｖ、印加時間；１０ｓｅｃとした。
【００５１】
除電ブラシ１５は、直径０．０３５ｍｍ、体積抵抗率０．０１〜１Ω・ｍｍの導電性繊維を６０本／束の単位で束にし、更にそれを図３に示すように、９．５束／１０ｍｍの密度のブラシ部１５ａとして、導電性の植毛部１５ｂの長手方向に並べたものである。除電ブラシ１５の短手方向の長さは、ブラシ部１５ａ及び植毛部１５ｂを含めて１５．０ｍｍで、本実施例ではブラシ部１５ａの長さが１０．０ｍｍである。長手方向の長さは中間転写ベルト８の移動方向と交差する方向の長さと同じである。
【００５２】
設置位置は、図４に示すように、駆動ローラ１１の中心Ｏを通る鉛直線を左方向に１．０ｍｍシフトした鉛直線が中間転写ベルト８、駆動ローラ１１と交わる点をＰ、Ｑとした時に、ブラシ部１５ａの先端中心部が、Ｐ、Ｑと共に１．０ｍｍの距離となるようにされており、電気的には接地されている。
【００５３】
尚、上記した除電ブラシ１５の設置位置は、上述したように、二次転写前帯電手段１４と対向する位置が望ましく、中間転写ベルト８と駆動ローラ１１との接点部分から、中間転写ベルト８移動方法下流側に、駆動ローラ１１の半径分の距離より近くの、つまり前記の図４に示した領域α内の位置に除電ブラシ１５の先端が存在するように配置されている。
【００５４】
そして、領域αである中間転写ベルト８と駆動ローラ１１との対向部に形成される空隙αに対して、ブラシ部１５ａの先端部を、駆動ローラ１１の回転や中間転写ベルト８の搬送に巻き込まれる恐れがなく、且つ駆動ローラ１１の表面や中間転写ベルト８の裏面に強く押し当たる恐れがない範囲内で、極力、中間転写ベルト８と駆動ローラ１１が離れる点の近傍に近づけたものである。
【００５５】
次に、上述した本実施例における除電ブラシ１５の効果を、以下の実験例にて説明する。
【００５６】
実験例１
比較例１である除電ブラシ１５の設置の無い画像形成装置と、上記に説明した本実施例１の除電ブラシ１５の設置の有る画像形成装置において、出力画像の画質の比較検討を行った。
【００５７】
表１に、上記の比較例１と本実施例の画像形成装置における画像の比較検討結果を示す。
【００５８】
【表１】

【００５９】
表１における画像ムラレベル（濃度ムラレベル）の判定は、目視による官能評価により行い、判定基準は、○＝発生無し、×＝発生が顕著で画質的に許容できない程度のレベルである。
【００６０】
上記の判定の結果、除電ブラシ１５の設置の無い比較例１の画像形成装置において、中間転写ベルト８と駆動ローラ１１の剥離放電に起因すると考えられる濃度ムラが、光学濃度０．３〜０．８（マクベス社製；ＲＤ９１８による測定）のハーフトーン画像上に顕著に発生した。一方、本実施例１における除電ブラシ１５の設置の有る画像形成装置では、濃度ムラは確認されなかった。
【００６１】
この濃度ムラの発生が、除電ブラシ１５の有り無しに依存するメカニズムは、以下のように考えられる。
【００６２】
濃度ムラの発生原因は、中間転写ベルト８と駆動ローラ１１との間で剥離放電が生じることによると考えられる。即ち、二次転写前帯電工程において、中間転写ベルト８上のトナーが帯電されてトリボ上昇を受ける際に、中間転写ベルト８自体も帯電を受け、一時的に電荷を保持する。
【００６３】
一方、本二次転写前帯電部の直下流部において、中間転写ベルト８が駆動ローラ１１より離れる部分においては、つまり図４に示す中間ベルト８と駆動ローラ１１との接点から駆動ローラ１１の半径距離分、中間転写ベルト８移動方向下流側の領域αにおいては、空隙αが形成され、空気層の急激な容量変化が生じる。従って、空隙部αには局所的な高電圧が発生することになり、中間転写ベルト８の裏面と駆動ローラ１１の表面との間には、剥離放電が生じる。
【００６４】
濃度ムラは、本放電の影響により、中間転写ベルト８の表面上のトナーが、その面内を瞬時に移動することによるものであると考えられる。
【００６５】
従って、空隙部α近辺にて、中間転写ベルト８をその裏面側より除電することを目的として、電気的に接地された除電ブラシ１５を設置した場合には、未然に剥離放電を抑制し，画像ムラの発生を防止する。
【００６６】
上記実験例１の比較検討結果より、除電ブラシ１５を設置することで、二次転写前帯電ムラに起因する画像ムラの発生を防止できる。
【００６７】
尚、前記した除電ブラシ１５の設置は、画像ムラを防止するためだけでなく、その他の異常放電に伴う画像不良のない良好な画像を得る上でも効果的である。即ち、中間転写ベルト８を二次転写前帯電工程の直後の部分において除電することにより、中間転写ベルト８が二次転写部Ｔｎ２に移動されるまでの間、近接する各部材との間で生じ得る種々の異常放電をも抑制し、本放電に起因し、ハーフトーンの画像上に生じる画像不良に関しても防止できるようになる。
【００６８】
しかし、本実施例以外の中間転写ベルト８や駆動ローラ１１を用いた画像形成装置に応用した場合、本実施例に用いられたものと同じ除電ブラシ１５を設置しているにも関わらず、二次転写前帯電手段１４による剥離放電が生じ、画像ムラが発生することがあった。従って、除電ブラシ１５の効果は、その除電対象である駆動ローラ１１及び中間転写ベルト８の性質に大きく依存していることが考えられた。
【００６９】
そこで、次に、上述した本実施例１における中間転写ベルト８、駆動ローラ１１の表面抵抗率に関して、以下の実験例２、３を元に詳細に説明する。
【００７０】
実験例２
比較例２、３、４、５である、表面抵抗率が１０^７Ω／□、１０^８Ω／□、１０^１０Ω／□、１０^１１Ω／□の各駆動ローラ１１を用いた以外は実施例１と同様の構成の画像形成装置と、本実施例１における表面抵抗率が１０^９Ω／□の駆動ローラ１１と、を用いた画像形成装置において、出力画像の画質の比較検討を行った。
【００７１】
表２に、上記の比較例２〜５と本実施例１における表面抵抗率の各駆動ローラによる画像の比較検討結果を示す。
【００７２】
【表２】

【００７３】
尚、表２における画像ムラレベル（濃度ムラレベル）の判定は、目視による官能評価により行い、判定基準は、○＝発生無し、△＝発生が軽微で実用上問題にならない程度のレベル、×＝発生が顕著で画質的に許容できない程度のレベルである。
【００７４】
本検討では、表面抵抗率が比較例４の１０^１０Ω／□と比較例５の１０^１１Ω／□の駆動ローラ１１の両者において、除電ブラシ１５を設置しているにも関わらず、光学濃度０．３〜０．８（マクベス社製；ＲＤ９１８による測定）のハーフトーン画像上の濃度ムラが発生した。但し、濃度ムラレベルとしては、表面抵抗率が１０^１０Ω／□の比較例４の駆動ローラ１１を用いたものの方が軽微であった。
【００７５】
一方、本実施例１における表面抵抗率の駆動ローラ１１、表面抵抗率が１０^７Ω／□の比較例１と１０^８Ω／□の比較例２の駆動ローラ１１では、濃度ムラは確認されなかった。
【００７６】
この濃度ムラの発生が、駆動ローラ１１の表面抵抗率に依存するメカニズムを，本発明者は以下のように考えている。
【００７７】
表面抵抗率の高い駆動ローラ１１を用いた場合、二次転写前帯電バイアス源１７から供給される電流が駆動ローラ１１に流れにくくなり、上記に説明した中間転写ベルト８と駆動ローラ１１との空隙αにて生じる剥離放電は、より微小な空隙α、即ち、二次転写前帯電部の直下流部で中間転写ベルト８と駆動ローラ１１が離れる点の極近傍部分で生じるようになる。
【００７８】
従って、前記した剥離放電は、本実施例１の除電ブラシ１５が中間転写ベルト８を除電する以前の段階で発生することになり、除電ブラシ１５による剥離放電の抑制効果は弱まる。そこで，濃度ムラが十分に防止できなくなる。
【００７９】
上記実験例２の比較検討結果より、駆動ローラ１１の表面抵抗率を１０^１０Ω／□以下とすることで、除電ブラシ１５による画像ムラの防止効果を維持できる。
【００８０】
又、駆動ローラ１１である二次転写前帯電手段対向部材の表面抵抗率は限界まで低くしてもよいので、特に設けない。ただし、本実施例のように、二次転写前帯電手段対向部材がベルトを駆動するための機能を有する場合には、ベルト裏面とのスリップ防止を目的としてゴムの被覆層が設けられるために材料的な抵抗下限が存在する。
【００８１】
実験例３
比較例６、７、８、９、１０として、裏面における表面抵抗率が、１０^６Ω／□、１０^８Ω／□、１０^１２Ω／□、１０^１４Ω／□、１０^１６Ω／□の各中間転写ベルト８と、本実施例１における裏面における表面抵抗率が１０^１０Ω／□の中間転写ベルト８と、を用いた画像形成装置において、出力画像の画質の比較検討を行った。
【００８２】
表３に、上記の比較例６〜１０と本実施例１に備えられた、各裏面における表面抵抗率の各中間転写ベルト８による画像の比較検討結果を示す。
【００８３】
【表３】

【００８４】
尚、上表における濃度ムラレベルと画像チリレベルの判定は、目視による官能評価により行い、判定基準は、○＝発生無し、△＝発生が軽微で実用上問題にならない程度のレベル、×＝発生が顕著で画質的に許容できない程度のレベルである。
【００８５】
本検討では、裏面における表面抵抗率が１０^１４Ω／□以上の比較例９、１０の中間転写ベルト８では、除電ブラシ１５を設置しているにも関わらず、上述した濃度ムラが発生することが確認された。
【００８６】
この濃度ムラも、上記実験例２におけるものと同様に、中間転写ベルト８の裏面における表面抵抗率が高くなり、剥離放電がより微小な空隙αで生じるようになるために、除電ブラシ１５による剥離放電の抑制効果が弱まることで発生していると考えられる。
【００８７】
又、裏面における表面抵抗率が１０^８Ω／□以下の比較例６、７の中間転写ベルトでは、一次転写工程において、トナーの飛び散りにより線画像や文字画像などがぼやけてしまう現象（以下、「画像チリ」と称す。）が顕著になることが確認された。
【００８８】
この画像チリが中間転写ベルト８の表面抵抗率に依存するメカニズムを，本発明者は以下のように考えている。
【００８９】
画像チリの発生原因は、一次転写部Ｔａ〜Ｔｄ前の空隙部におけるトナー飛翔であると考えられる。よって、中間転写ベルト８の裏面における表面抵抗率が低いと、裏面における表面抵抗率が高い場合に比べて、一次転写部前後の空隙部にかかる電界が強くなるために、一次転写部Ｔａ〜Ｔｄの上流側からトナーが感光ドラム２ａ、２ｂ、２ｃ、２ｄから中間転写ベルト８上への飛翔が始まる。このために、中間転写ベルト８の裏面における表面抵抗率が低いほど画像チリが悪化していると考えられる。
【００９０】
上記実験例３の比較検討結果より、中間転写ベルト８の表面抵抗率を１０^８Ω／□以上１０^１４Ω／□以下とすることで、除電ブラシ１５による画像ムラの防止効果を維持できると共に画像チリの発生も防止できる。
【００９１】
上記の実験例１〜３の結果から考察して、中間転写ベルト８を有する画像形成装置において、中間転写ベルト８に一次転写されたトナー像を、二次転写部Ｔｎ２に達する前に二次転写前帯電手段１４によって帯電し、トナー像を構成するトナーの電荷付与によりトリボ値の均一化及び増加がなされる画像形成装置において、本実施例では、駆動ローラ１１である、二次転写前帯電手段１４の対向電極を構成する二次転写前帯電手段対向部材の直下流に除電ブラシ１５を設置し、駆動ローラ１１の少なくとも、中間転写ベルト８に接触又は近接する部分の表面抵抗率を、１０^１０Ω／□以下、及び中間転写ベルト８のトナー像が形成されない方の裏面の表面抵抗率を、１０^８Ω／□以上１０^１４Ω／□以下と規定することにより、二次転写前帯電に起因する画像の濃度ムラの発生を防止することができる。中間転写ベルト８の裏面及び駆動ローラ１１の表面抵抗率の測定方法は実施例１にてなされた方法で測定される。
【００９２】
尚、本発明は、画像形成装置の全体構成は、本実施例のものに限定されるものではなく、中間転写体移動方向で二次転写前帯電手段対向部材の直下流部で且つ中間転写体の裏面側に設置され、中間転写体を除電する除電手段を有するものに関し、二次転写前帯電手段対向部材の表面抵抗率を１０^１０Ω／□以下、中間転写体の裏面における表面抵抗率を、１０^８Ω／□以上１０^１４Ω／□以下の範囲に定めた、あらゆる形態の画像形成装置に適用できる。
【００９３】
実施例２
本発明の実施例２に係る画像形成装置は、図１に示した実施例１の電子写真方式のプリンタ等のフルカラー画像形成装置と同様である。従って、上記した実施例１における説明と重複する説明は省略する。
【００９４】
本実施例では、二次転写前帯電後の中間転写ベルト８を除電するための除電手段１５として、実施例１で採用した除電ブラシ１５ではなく、針状部材である除電針１５’が使用されている。
【００９５】
除電針１５’は、厚み０．１ｍｍ、短手方向の長さ１５．０ｍｍのＳＵＳ板長手方向に、図５に示すように、高さ３ｍｍ、ピッチ２ｍｍ、先端エッジＲ０．１ｍｍ以下の針形状の山を形成したものである。長手方向の長さは中間転写ベルト８の移動方向と交差する方向の長さと同じである。
【００９６】
設置位置は、図４に示した実施例１の除電ブラシ１５におけるものと同様であり、やはり、除電針１５’の先端中心部が、Ｐと１．０ｍｍの距離となるようにされており、電気的には接地されている。
【００９７】
本除電針１５’を用いた場合においても、中間転写ベルト８と駆動ローラ１１との間で生じる剥離放電を効果的に抑制することができ、二次転写前帯電に起因する画像の濃度ムラの発生を防止することができる。
【００９８】
尚、本除電針１５’は、除電ブラシ１５に比べ、硬度が硬く、丈夫であるため、長期にわたる連続プリント動作中においても変形する恐れがなく、耐久性に優れる等のメリットを有する。
【００９９】
一方で、実施例１で用いられた除電ブラシ１５は、細かい先端形状を有するブラシの一本一本が高密度で並べ合わせられているものであるため、本実施例における除電針１５’に比べ、除電性能が高く、安定している等のメリットを有する。
【０１００】
以上のように、実施例１同様に、中間転写ベルトを有する画像形成装置において、中間転写ベルトに一次転写されたトナー像を、二次転写部に達する前に二次転写前帯電手段によって帯電し、トナー像を構成するトナーの電荷付与によりトリボ値の均一化及び増加がなされる画像形成装置において、二次転写前帯電手段の対向電極を構成する二次転写前帯電手段対向部材の直下流に設置する除電部材として、除電ブラシに代わって、除電針を使用した場合においても、二次転写前帯電手段対向部材の少なくとも中間転写体に接触又は近接する部分の表面抵抗率を、１０^１０Ω／□以下、及び中間転写体のトナー像が形成される側に対して裏面の表面抵抗率を、１０^８Ω／□以上１０^１４Ω／□以下と規定することにより、二次転写前帯電に起因する画像の濃度ムラの発生を防止することができる。
【０１０１】
尚、除電部材としては、実施例１の除電ブラシや実施例２の除電針に限定されず、その寸法や材質等の構成は適宜決定される。
【０１０２】
【発明の効果】
以上説明したように、本発明の画像形成装置は、像担持体から現像剤像が一次転写される中間転写体を有し、中間転写体上に一次転写された現像剤像を二次転写部にて転写材に二次転写して画像を得る画像形成装置であって、更に、中間転写体の移動方向に対して、一次転写部の下流で且つ二次転写部の上流の位置且つ中間転写体の現像剤像担持面側の位置に設置され、現像剤像を、現像剤像の帯電極性と同極性に帯電する二次転写前帯電手段と、中間転写体を挟んで二次転写前帯電手段と対向する位置に設置され、中間転写体の現像剤担持面の裏面に接触又は近接する二次転写前帯電手段対向部材と、を有し、更に、中間転写体の移動方向に対して二次転写前帯電手段対向部材の直下流部であり且つ中間転写体の現像剤像担持面の裏面側に設置され、中間転写体を除電する除電手段を有し、二次転写前帯電手段対向部材の少なくとも中間転写体との接触部分の表面抵抗率が、１０^１０Ω／□以下であり、中間転写体の現像剤担持面の裏面の表面抵抗率が、１０^８Ω／□以上、１０^１４Ω／□以下の範囲である、ことにより、中間転写体と二次転写前帯電手段対向部材との間で剥離放電が生じることを抑制し、濃度ムラの発生を防止して良好な画像を提供できる、といった効果が得られる。
【図面の簡単な説明】
【図１】本発明に係る画像形成装置の一例を示す概略構成図である。
【図２】本発明に係る二次転写前帯電手段対向部材の一例である駆動ローラを示す断面図である。
【図３】本発明に係る除電部材の一例である除電ブラシを示す平面図である。
【図４】本発明に係る除電部材の配置を示す概略構成図である。
【図５】本発明に係る除電部材の他の例である除電針を示す平面図である。
【符号の説明】
１Ｙ、１Ｍ、１Ｃ、１Ｄ　　　画像形成部
２ａ、２ｂ、２ｃ、２ｄ　　　感光ドラム（像担持体）
５ａ、５ｂ、５ｃ、５ｄ　　　一次転写ローラ
８　　　　　　　　　　　　　中間転写ベルト（中間転写体）
１１　　　　　　　　　　　　駆動ローラ（二次転写前帯電手段対向部材）
１２　　　　　　　　　　　　二次転写対向ローラ（ローラ部材）
１３　　　　　　　　　　　　従動ローラ（ローラ部材）
１４　　　　　　　　　　　　二次転写前帯電手段
１５　　　　　　　　　　　　除電ローラ（除電部材）
１５’　　　　　　　　　　　除電針（除電部材）
１７　　　　　　　　　　　　二次転写前帯電バイアス源
１９　　　　　　　　　　　　二次転写ローラ
Ｔａ、Ｔｂ、Ｔｃ、Ｔｄ　　　一次転写部
Ｔｎ２　　　　　　　　　　　二次転写部[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an image forming apparatus such as a copying machine, a printer, and a facsimile that forms an image by an electrophotographic method or an electrostatic recording method.
[0002]
[Prior art]
As an electrophotographic multi-color or full-color image forming apparatus, a plurality of drum-shaped photoconductors (photosensitive drums) are arranged as image carriers according to each color, and developer images of each color formed on each photoconductor drum 2. Description of the Related Art An image forming apparatus for forming a color image by successively superposing (toner images) on an intermediate transfer member, particularly a belt-shaped intermediate transfer belt, that is, an image forming apparatus of a so-called in-line type / intermediate transfer system has been put to practical use. .
[0003]
Such an intermediate transfer type image forming apparatus is, for example, a transfer apparatus as described in Patent Literature 1, that is, a transfer body on which an image is formed is stuck or adsorbed on a transfer drum, and In comparison with a color electrophotographic apparatus that transfers an image from a latent image carrier such as a photosensitive drum, a color electrophotographic apparatus using the intermediate transfer body is capable of processing and controlling a transfer material (for example, gripping with a gripper, Since the image can be transferred from the intermediate transfer member without the need for adsorbing, giving a curvature, etc., thin paper (40 g / m2, such as envelopes, postcards, label paper, etc.) can be used. ² Paper) to thick paper (200g / m ² This has the advantage that a wide variety of transfer materials can be selected irrespective of the width, the length, or the thickness of the paper.
[0004]
Further, by using a belt-like intermediate transfer body, the degree of freedom in arranging the intermediate transfer body inside the image forming apparatus is increased as compared with the case where a rigid cylinder such as an intermediate transfer drum is used, and the space is effectively used. There is also an advantage that the size and cost of the apparatus main body can be reduced.
[0005]
In the above-described conventional in-line type color image forming apparatus, the intermediate transfer belt is in contact with or close to each photosensitive drum at the respective primary transfer portions, and the toner image formed on each photosensitive drum is placed on the intermediate transfer belt. By successively superimposing and transferring (primary transfer), for example, four color toner images of yellow, magenta, cyan, and black are formed on the intermediate transfer belt. The color toner image formed on the intermediate transfer belt is carried on the intermediate transfer belt, and is transferred from the intermediate transfer belt to the transfer material at the secondary transfer section where the transfer material is transported and comes into contact with the intermediate transfer belt. Thereafter, the transfer material onto which the unfixed toner image has been transferred reaches the fixing device, where the transfer material is heated and pressed by the fixing device to obtain a permanent fixed image.
[0006]
Here, at a position downstream of the primary transfer section and upstream of the secondary transfer section in the direction of movement of the intermediate transfer belt, a secondary transfer pre-charging means for charging the toner image to the same polarity as the toner image faces the intermediate transfer belt surface. Before the secondary transfer to the transfer material such as paper, the color toner image formed on the intermediate transfer belt is (Toner).
[0007]
By applying the electric charge of the toner on the surface of the intermediate transfer belt by the pre-secondary transfer charging means, the toner tribo of each color is made uniform and the tribo value is increased, and the subsequent secondary transfer efficiency is improved.
[0008]
Here, the secondary transfer efficiency is the efficiency with which the toner on the intermediate transfer belt is transferred onto the transfer material in the secondary transfer step.
[0009]
[Patent Document 1]
JP-A-63-301960
[Patent Document 2]
JP-A-2000-28461
[0010]
[Problems to be solved by the invention]
However, in the electrophotographic image forming apparatus having the above-mentioned intermediate transfer belt, while the secondary transfer efficiency is improved by the pre-secondary transfer charging by the pre-secondary transfer charging means, this secondary transfer efficiency is improved. There is a problem that image density unevenness easily occurs due to charging.
[0011]
The image unevenness due to the pre-secondary transfer charging means that immediately after the pre-secondary transfer charging step, the intermediate transfer belt and the pre-secondary transfer charging unit facing member used as a counter electrode of the pre-secondary transfer charging unit. It is considered that the peeling discharge occurs between the first and second transfer belts, and the toner on the surface of the intermediate transfer belt moves in the plane due to the influence of the peeling discharge, thereby causing unevenness in image density.
[0012]
Accordingly, an object of the present invention is to provide an intermediate transfer member on which a developer image is primarily transferred from an image carrier on which a developer image is formed, and to transfer the developer image on the surface of the primary transferred intermediate transfer member. It is an object of the present invention to provide an image forming apparatus for performing secondary transfer to a material, in which the occurrence of image density unevenness due to charging of a developer image from an intermediate transfer member to a transfer material before secondary transfer is prevented.
[0013]
[Means for Solving the Problems]
The above object is achieved by an image forming apparatus according to the present invention. In summary, the present invention has an image carrier on which a developer image is formed on the surface, and an intermediate transfer body on which the developer image is primarily transferred from the image carrier at a primary transfer unit, An image forming apparatus that obtains an image by secondary-transferring the developer image primarily transferred onto the intermediate transfer body to a transfer material at a secondary transfer unit,
Further, the intermediate transfer member is provided at a position downstream of the primary transfer portion and upstream of the secondary transfer portion with respect to a moving direction of the intermediate transfer member and on a side of the developer image bearing surface of the intermediate transfer member. A pre-secondary transfer charging unit that charges the developer image carried on the surface to the same polarity as the charging polarity of the developer image; and a position facing the pre-secondary transfer charging unit with the intermediate transfer member interposed therebetween. In the image forming apparatus, the charging device facing the secondary transfer pre-transfer member contacting or close to the back surface of the developer carrying surface of the intermediate transfer body,
Further, the intermediate transfer member is disposed immediately downstream of the member facing the charging means before the secondary transfer with respect to the moving direction of the intermediate transfer member and on the back side of the developer image bearing surface of the intermediate transfer member. Having a static elimination means for eliminating static electricity,
The surface resistivity of at least the portion of the pre-secondary transfer charging member facing member that is in contact with the intermediate transfer member is 10%. ¹⁰ Ω / □ or less,
The surface resistivity of the back surface of the developer carrying surface of the intermediate transfer member is 10 ⁸ Ω / □ or more, 10 ¹⁴ Ω / □ or less,
An image forming apparatus is provided.
[0014]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, the image forming apparatus according to the present invention will be described in more detail with reference to the drawings.
[0015]
Example 1
FIG. 1 is a schematic configuration diagram illustrating a full-color image forming apparatus such as an electrophotographic printer, which is an image forming apparatus according to a first embodiment of the present invention.
[0016]
The image forming apparatus includes an image forming unit 1Y for forming a yellow image, an image forming unit 1M for forming a magenta image, an image forming unit 1C for forming a cyan image, and a black image. There are four image forming units (image forming units) of the image forming unit 1K to be formed, and these four image forming units are arranged in a line at a fixed interval.
[0017]
In each of the

image forming units

1Y, 1M, 1C, and 1K, photosensitive drums 2a, 2b, 2c, and 2d as image carriers are provided, respectively. Around the photosensitive drums 2a, 2b, 2c, 2d, charging

rollers

3a, 3b, 3c, 3d, developing

devices

4a, 4b, 4c, 4d,

transfer rollers

5a, 5b, 5c, 5d, a drum cleaning device 6a,

Exposure devices

7a, 7b, 7c, and 7d are respectively installed above the charging

rollers

3a, 3b, 3c, and 3d and the developing

devices

4a, 4b, 4c, and 4d. I have.
[0018]
In the present embodiment, the photosensitive drums 2a to 2d are negatively charged organic photosensitive drums having an outer diameter of 30.0 mm, have an OPC photosensitive layer on a drum base made of aluminum or the like, and are driven by a driving device (not shown). It is rotationally driven at a predetermined peripheral speed (process speed) in each of the directions (counterclockwise).
[0019]
The charging rollers 3a to 3d serving as contact charging means contact the photosensitive drums 2a to 2d with a predetermined pressing force, and the surfaces of the photosensitive drums 2a to 2d are negatively charged by a charging bias applied from a charging bias power supply (not shown). Is uniformly charged to a predetermined potential.
[0020]
In the present embodiment, the developing devices 4a to 4d are of a two-component developing type, and develop an electrostatic latent image formed on each of the photosensitive drums 2a to 2d by a developing bias applied from a developing bias power supply (not shown). A developer (toner) is adhered and reversal development is performed as a developer image (toner image). The developing

devices

4a, 4b, 4c, and 4d store yellow toner, cyan toner, magenta toner, and black toner, respectively, as developers.
[0021]
The primary transfer rollers 5a to 5d as contact transfer means make contact with the surfaces of the photosensitive drums 2a to 2d with a predetermined pressing force via an intermediate transfer belt 8 as an intermediate transfer member, and transfer bias power supplies 9a, 9b, 9c. , 9d, the primary transfer portions Ta, Tb, Tc, and Td, which are the nip portions between the photosensitive drums 2a to 2d and the transfer rollers 5a to 5d, by the transfer bias having a polarity opposite to that of the toner. The toner images of the respective colors on the surfaces 2a to 2d are sequentially superimposed on the moving intermediate transfer belt 8 and transferred (primary transfer).
[0022]
The exposure devices 7a to 7d perform image exposure on the surfaces of the photosensitive drums 2a to 2d with laser light modulated according to time-series electric digital pixel signals of image information input from a host computer (not shown). Then, an electrostatic latent image corresponding to the image information is formed on the surfaces of the photosensitive drums 2a to 2d charged by the charging rollers 3a to 3d.
[0023]
In the configuration of the present embodiment, the intermediate transfer belt 8 is stretched by three roller members of a driving roller 11, a secondary transfer opposing roller 12, and a driven roller 13, and the photosensitive drum is rotated by the driving roller 11. It is moved (rotated) in the direction of the arrow in synchronization with the rotation of 2a to 2d. In addition, a discharging member (a discharging brush) 15 which is a brush-like member is disposed immediately downstream of the driving roller 11.
[0024]
The details of the intermediate transfer belt 8, the drive roller 11, and the charge removal brush 15, which are features of the present invention, will be described later.
[0025]
It is formed on the downstream side of the primary transfer portions Ta to Td in contact with the most downstream photosensitive drum 2d with respect to the moving direction of the intermediate transfer belt 8, and between the secondary transfer opposing roller 12 and the secondary transfer roller 19. In the vicinity of the drive roller 11 located on the upstream side of the secondary transfer portion Tn2, a pre-secondary transfer charging unit 14 is provided. Further, a belt cleaning device 16 is installed near the surface of the intermediate transfer belt 8 downstream of the secondary transfer portion Tn2 between the secondary transfer opposing roller 12 and the driven roller 13.
[0026]
Next, an image forming operation of the image forming apparatus of the present embodiment will be described.
[0027]
When the image forming operation start signal is issued, each of the photosensitive drums 2a to 2d of each of the image forming units 1Y to 1K, which is driven to rotate at a predetermined process speed (117 mm / sec in this embodiment), is charged with a charging roller 3a to 3d. By 3d, it is uniformly charged to a predetermined potential (about -650 V in this embodiment) which is negative in this embodiment.
[0028]
The exposure devices 7a to 7d respectively convert the color-separated image signals input from the host computer (not shown) into optical signals, and convert the converted optical signals, ie, laser beams, into

mirrors

10a, 10b. , 10c, and 10d, respectively, and scans and exposes each of the charged photosensitive drums 2a to 2d to form an electrostatic latent image.
[0029]
Then, first, a developing bias having the same polarity as the charging polarity (negative polarity) of the photosensitive drum 2a is applied to the electrostatic latent image formed on the photosensitive drum 2a by the developing device 4a, so that the yellow stored in the developing device 4a is applied. The toner is adhered and reversal development is performed to make a visible image as a toner image.
[0030]
The developing bias in this embodiment was a DC component: -400 V, an AC component: 1.5 kVpp, a frequency: 3 kHz, a waveform: a rectangular wave, which is a bias obtained by superimposing an AC voltage on a DC voltage.
[0031]
The yellow toner image is moved (rotated) in the primary transfer section Ta by the primary transfer roller 5a to which a primary transfer bias (in this embodiment, a constant current control of +15 μA) is applied from the transfer bias power supply 9a. The primary transfer is performed on the intermediate transfer belt 8.
[0032]
In the intermediate transfer belt 8 to which the yellow toner image has been transferred, the position portion of the image forming portion 1Y moves to the image forming portion 1M side by the driving of the driving roller 11. In the image forming section 1M, the magenta toner image similarly formed on the photosensitive drum 2b is also transferred to the primary transfer section Tb by the primary transfer roller 5b to which a primary transfer bias is applied from the primary transfer bias power supply 9b. The image is superimposed and transferred on the yellow toner image on the intermediate transfer belt 8.
[0033]
Hereinafter, cyan and black toner images formed by the photosensitive drums 2c and 2d of the image forming units 1C and 1K are sequentially transferred onto the yellow and magenta toner images superimposedly transferred on the intermediate transfer belt 8 in the same manner. The

primary transfer rollers

5c and 5d to which the primary transfer bias is applied from the respective transfer bias power sources 9c and 9d at the respective primary transfer portions Tc and Td are sequentially superimposed and transferred to transfer a full-color toner image onto the intermediate transfer belt 8. Form.
[0034]
Prior to the secondary transfer, the full-color toner image formed on the intermediate transfer belt 8 has the same polarity (negative polarity) as the toner from the secondary transfer pre-charging means 14 which is a corona charger in this embodiment. Receives charge. The pre-secondary transfer charging means 14 is installed opposite to the driving roller 11 with the intermediate transfer belt 8 interposed therebetween, and is biased by a pre-secondary transfer charging bias source 17 (in this embodiment, DC: -2.5 kV, AC: 4.0 kVpp), and charges the toner image formed on the surface of the intermediate transfer belt 8 to the same polarity (negative polarity).
[0035]
In the corona charger of the present embodiment, approximately one fifth of the total current (−110 μA in RMS value) supplied from the pre-secondary transfer bias source 17 is charged by the pre-secondary transfer charging unit 14. Flows to the drive roller 11 which is the opposite member of.
[0036]
Due to the charging before the secondary transfer, the toner tribo on the intermediate transfer belt 8 rises from about −20 μC / g before the charging to about −35 μC / g after the charging.
[0037]
Then, the transfer of the conveyed paper or the like is performed in accordance with the timing at which the leading end of the full-color toner image on the intermediate transfer belt 8 moves to the secondary transfer portion Tn2 between the secondary transfer roller 19 and the secondary transfer opposing roller 12. The material P is transported to the secondary transfer portion Tn2, and is transferred by the secondary transfer roller 19 to which a secondary transfer bias (in this embodiment, +1.7 kV) having a polarity opposite to that of the toner is applied from the secondary transfer bias power supply 20. Then, a full-color toner image is collectively secondary-transferred onto the transfer material P.
[0038]
Then, the transfer material P on which the full-color toner image is formed is conveyed to the fixing device 21, and the full-color toner image is transferred by heating and pressing at a fixing nip portion between the fixing roller 21a and the pressure roller 21b of the fixing device 21. After being thermally fixed on the surface of the material P, the material P is discharged to the outside, and a series of image forming operations is completed.
[0039]
In the above-described primary transfer step, transfer residual toner remaining on the photosensitive drums 2a to 2d is removed and recovered by the drum cleaning devices 6a to 6d, respectively. Further, residual toner remaining on the surface of the intermediate transfer belt 8 after the secondary transfer is removed and collected by the belt cleaning device 16. The cleaned intermediate transfer belt 8 is ready for the next image formation.
[0040]
As described above, in the present configuration, the full-color toner image primary-transferred and formed on the intermediate transfer belt 8 is transferred from the corona charger as the pre-secondary transfer charging unit 14 with the toner before the secondary transfer. Upon receiving the charge of the same polarity (negative polarity), the toner tribo on the intermediate transfer belt 8 is increased from about -20 μC / g to about −35 μC / g. Therefore, the secondary transfer efficiency can be increased. However, in such an image forming apparatus, as described in the related art, the secondary roller which is used as the counter electrode of the pre-secondary transfer charging unit 14 and is the drive roller 11 in this case. Separation discharge occurs between the pre-transfer charging member facing member and causes image unevenness.
[0041]
Here, since the pre-secondary-transfer charging means 14 is installed so as to face the drive roller 11 with the intermediate transfer belt 8 interposed therebetween, the pre-secondary-transfer-charge-medium-facing member is the drive roller 11. . Then, about ５ of the total current supplied from the pre-secondary transfer charging bias source 17 flows to the drive roller 11. Needless to say, the member facing the charging means before the secondary transfer does not have to be a roller for driving the intermediate transfer belt, and is independent of the roller member on which the intermediate transfer belt 8 is stretched. It may be provided inside.
[0042]
Therefore, in the present embodiment, the pre-secondary transfer charging means 14 is brought into contact with or close to the back surface of the toner image bearing surface of the intermediate transfer belt 8 and charges the same polarity as the charging polarity of the toner image with the intermediate transfer belt 8 interposed therebetween. The driving roller 11, which is an opposing member of the pre-secondary transfer charging means, and a portion immediately downstream of the driving roller 11 with respect to the moving direction of the intermediate transfer belt 8 and on the back side of the toner image bearing surface of the intermediate transfer belt 8. And an electricity removing means for removing electricity from the intermediate transfer belt 8, in this embodiment, an electricity removing brush 15.
[0043]
The discharge brush 15 removes the discharge between the intermediate transfer belt 8 and the drive roller 11 to remove the discharge, thereby suppressing the occurrence of the discharge.
[0044]
It is desirable that the charge removing member such as the charge removing brush 15 be located as close as possible to the member facing the pre-secondary transfer charging means such as the drive roller 11, and be located at a position facing the charging means 14 before the secondary transfer. desirable. In this embodiment, as shown in FIG. 4, the neutralization brush 15 is moved from the contact point between the intermediate transfer belt 8 and the drive roller 11 to the downstream side of the intermediate transfer belt 8 moving method by the distance of the radius of the drive roller 11. At a closer distance, the tip of the charge removing brush 15 is arranged so as to be present in a gap α between the driving roller 11 and the intermediate transfer belt 8, which is indicated by a hatched area in FIG.
[0045]
As the intermediate transfer belt 8 used here, a resin such as a urethane-based resin, a fluorine-based resin, a nylon-based resin, or a polyimide resin, or a material obtained by dispersing carbon or conductive powder in these materials and adjusting the resistance is used. Can be. Further, as described in Patent Document 2, the control of the primary transfer and the secondary transfer bias is simplified by using an intermediate transfer belt having a short charging relaxation time and having a so-called self-decay type electrical characteristic. And a good full-color image can be stably obtained.
[0046]
In this embodiment, as the intermediate transfer belt 8, carbon is dispersed in polyimide so that both the surface resistivity on the front side and the surface resistivity on the back side are 10%. ¹⁰ A single-layer endless belt having a circumference of 1000 mm and a thickness of 100 μm, which was adjusted to Ω / □ and had a short charge relaxation time and so-called self-decaying electrical characteristics, was used.
[0047]
The measurement of the surface resistivity is based on JIS-K6911. After obtaining good contact between the electrode and the belt surface by using a conductive rubber as an electrode, a resistance meter (R8340, manufactured by Advantest) is used. Measured. The measurement conditions were: applied voltage; 100 V, applied time: 10 sec.
[0048]
The intermediate transfer belt 8 is stretched by three roller members including a drive roller 11, a secondary transfer opposed roller 12, and a driven roller 13 included in the intermediate transfer belt 8, and the drive roller 11, the secondary transfer opposed roller 12, the driven roller 13 is electrically grounded. The number and type of the roller members are not limited to the above.
[0049]
As shown in FIG. 2, the drive roller 11 serving as a member facing the charging means before the secondary transfer is composed of an aluminum core bar 11a having a diameter of 24.0 mm and a hydrin rubber layer 11b having a thickness of 2.9 mm. The roller has a surface resistivity of 10% by adjusting the resistance of hydrin rubber. ⁹ Ω / □. The hardness of the drive roller 11 is 40 ° in JIS-A standard.
[0050]
The measurement of the surface resistivity is based on JIS-K6911. A good contact between the electrode and the roller surface is obtained by pressing a metal probe electrode against the roller surface, and then a resistivity meter (manufactured by Mitsubishi Yuka Co., Ltd.) ; Hiresta IP MCP-HT250). The measurement conditions were: applied voltage; 10 V, applied time: 10 sec.
[0051]
The static elimination brush 15 bundles conductive fibers having a diameter of 0.035 mm and a volume resistivity of 0.01 to 1 Ω · mm in units of 60 fibers / bundle, and further, as shown in FIG. The brush portions 15a having a density of 10 mm are arranged in the longitudinal direction of the conductive flocking portion 15b. The length of the neutralization brush 15 in the width direction is 15.0 mm including the brush portion 15a and the flocked portion 15b. In the present embodiment, the length of the brush portion 15a is 10.0 mm. The length in the longitudinal direction is the same as the length in the direction intersecting the moving direction of the intermediate transfer belt 8.
[0052]
As shown in FIG. 4, points P and Q are points at which a vertical line obtained by shifting the vertical line passing through the center O of the drive roller 11 to the left by 1.0 mm to the intermediate transfer belt 8 and the drive roller 11 as shown in FIG. Occasionally, the center of the tip of the brush portion 15a has a distance of 1.0 mm together with P and Q, and is electrically grounded.
[0053]
As described above, the installation position of the above-described neutralization brush 15 is desirably at a position facing the pre-secondary transfer charging means 14, and the intermediate transfer belt 8 moves from the contact point between the intermediate transfer belt 8 and the driving roller 11. The discharge brush 15 is disposed on the downstream side of the method so that the tip of the discharge brush 15 is located at a position closer to the radius of the drive roller 11, that is, in the area α shown in FIG.
[0054]
Then, the tip of the brush portion 15a is caught in the rotation of the drive roller 11 and the conveyance of the intermediate transfer belt 8 in the gap α formed in the area α between the intermediate transfer belt 8 and the drive roller 11. This is as close as possible to the vicinity of the point where the intermediate transfer belt 8 and the driving roller 11 are separated from each other as long as there is no risk of being pressed against the front surface of the driving roller 11 or the back surface of the intermediate transfer belt 8. .
[0055]
Next, the effect of the neutralization brush 15 in the above-described embodiment will be described with reference to the following experimental examples.
[0056]
Experimental example 1
The image forming apparatus without the static elimination brush 15 of Comparative Example 1 and the image forming apparatus with the static elimination brush 15 of the first embodiment described above were compared and examined for the image quality of the output image.
[0057]
Table 1 shows the results of a comparative study of the images in the image forming apparatuses of Comparative Example 1 and the present embodiment.
[0058]
[Table 1]

[0059]
The determination of the image unevenness level (density unevenness level) in Table 1 is performed by visual sensory evaluation, and the criteria are as follows: o = no occurrence, x = remarkable occurrence and unacceptable in terms of image quality.
[0060]
As a result of the above determination, in the image forming apparatus of Comparative Example 1 in which the static elimination brush 15 was not installed, the density unevenness considered to be caused by the peeling discharge between the intermediate transfer belt 8 and the driving roller 11 was reduced to the optical density of 0.3 to 0.1. 8 (manufactured by Macbeth; measured by RD918). On the other hand, in the image forming apparatus having the static elimination brush 15 according to the first embodiment, no density unevenness was observed.
[0061]
The mechanism by which the occurrence of the density unevenness depends on the presence or absence of the charge removing brush 15 is considered as follows.
[0062]
It is considered that the cause of the density unevenness is that peeling discharge occurs between the intermediate transfer belt 8 and the driving roller 11. That is, in the pre-secondary transfer charging step, when the toner on the intermediate transfer belt 8 is charged and undergoes a tribo rise, the intermediate transfer belt 8 itself is also charged and temporarily holds the charge.
[0063]
On the other hand, in a portion immediately downstream of the charging section before the secondary transfer, in a portion where the intermediate transfer belt 8 is separated from the drive roller 11, that is, a radius of the drive roller 11 from a contact point between the intermediate belt 8 and the drive roller 11 shown in FIG. In the area α on the downstream side in the moving direction of the intermediate transfer belt 8 by the distance, a gap α is formed, and a rapid change in capacity of the air layer occurs. Accordingly, a local high voltage is generated in the gap α, and a separation discharge occurs between the back surface of the intermediate transfer belt 8 and the surface of the drive roller 11.
[0064]
It is considered that the density unevenness is caused by the instantaneous movement of the toner on the surface of the intermediate transfer belt 8 on the surface due to the influence of the main discharge.
[0065]
Therefore, in the case where an electrically grounded neutralizing brush 15 is provided in order to neutralize the intermediate transfer belt 8 from the back side in the vicinity of the gap α, the peeling discharge is suppressed beforehand and the image is removed. Prevent the occurrence of unevenness.
[0066]
According to the result of the comparative study of Experimental Example 1, it is possible to prevent the occurrence of image unevenness due to the charging unevenness before the secondary transfer by installing the charge removing brush 15.
[0067]
The installation of the charge removing brush 15 is effective not only for preventing image unevenness, but also for obtaining a good image without image defects due to other abnormal discharge. That is, by discharging the intermediate transfer belt 8 in a portion immediately after the pre-secondary transfer charging step, the intermediate transfer belt 8 is generated between adjacent members until the intermediate transfer belt 8 is moved to the secondary transfer portion Tn2. It is also possible to suppress various abnormal discharges to be obtained, and to prevent an image defect caused on a halftone image due to the main discharge.
[0068]
However, when the present invention is applied to an image forming apparatus using the intermediate transfer belt 8 and the driving roller 11 other than the present embodiment, the same static elimination brush 15 as that used in the present embodiment is installed. In some cases, peeling discharge was caused by the pre-transfer charging unit 14 and image unevenness occurred. Therefore, it is considered that the effect of the charge removing brush 15 largely depends on the properties of the drive roller 11 and the intermediate transfer belt 8 to be removed.
[0069]
Then, the surface resistivity of the intermediate transfer belt 8 and the drive roller 11 in the above-described first embodiment will be described in detail based on Experimental Examples 2 and 3 below.
[0070]
Experimental example 2
Comparative Examples 2, 3, 4, and 5 having a surface resistivity of 10 ⁷ Ω / □, 10 ⁸ Ω / □, 10 ¹⁰ Ω / □, 10 ¹¹ An image forming apparatus having the same configuration as that of the first embodiment except that each of the driving rollers 11 of Ω / □ was used, and the surface resistivity in the first embodiment was 10 ⁹ In an image forming apparatus using the Ω / □ drive roller 11 and the image forming apparatus using the Ω / □, a comparative study of the image quality of the output image was performed.
[0071]
Table 2 shows the results of a comparative study of the images obtained by the respective driving rollers of the surface resistivity in Comparative Examples 2 to 5 and Example 1.
[0072]
[Table 2]

[0073]
The determination of the image unevenness level (density unevenness level) in Table 2 was performed by visual sensory evaluation. The criteria were as follows: o = no occurrence, Δ = a level that occurred little and did not pose a problem in practice, and x = occurrence The level is remarkable and unacceptable in image quality.
[0074]
In this study, the surface resistivity was 10% in Comparative Example 4. ¹⁰ Ω / □ and 10 of Comparative Example 5 ¹¹ Density unevenness on a halftone image with an optical density of 0.3 to 0.8 (manufactured by Macbeth Co .; measured by RD918) despite the installation of the neutralization brush 15 on both the Ω / □ drive rollers 11. There has occurred. However, as the density unevenness level, the surface resistivity is 10 ¹⁰ The roller using the driving roller 11 of Comparative Example 4 of Ω / □ was slightly lighter.
[0075]
On the other hand, the driving roller 11 having a surface resistivity of 10 ⁷ Comparative Examples 1 and 10 of Ω / □ ⁸ In the drive roller 11 of Comparative Example 2 of Ω / □, density unevenness was not confirmed.
[0076]
The present inventor considers the mechanism by which the occurrence of the density unevenness depends on the surface resistivity of the drive roller 11 as follows.
[0077]
When the drive roller 11 having a high surface resistivity is used, the current supplied from the pre-secondary transfer charging bias source 17 does not easily flow to the drive roller 11, and the gap between the intermediate transfer belt 8 and the drive roller 11 described above is formed. The peeling discharge generated at α is generated in a minute gap α, that is, a very close portion where the intermediate transfer belt 8 and the driving roller 11 are separated from each other immediately downstream of the pre-secondary transfer charging unit.
[0078]
Therefore, the above-described peeling discharge occurs before the neutralizing brush 15 of the first embodiment neutralizes the intermediate transfer belt 8, and the effect of suppressing the peeling discharge by the neutralizing brush 15 is weakened. Therefore, density unevenness cannot be sufficiently prevented.
[0079]
From the result of the comparative study of Experimental Example 2, the surface resistivity of the drive roller 11 was set to 10 ¹⁰ By setting the resistance to Ω / □ or less, the effect of preventing the image unevenness by the charge removing brush 15 can be maintained.
[0080]
In addition, the surface resistivity of the member facing the charging means before the secondary transfer, which is the driving roller 11, may be reduced to the limit, and is not particularly provided. However, in the case where the member facing the charging means before the secondary transfer has a function for driving the belt as in this embodiment, the material is provided because the rubber coating layer is provided for the purpose of preventing slippage with the back surface of the belt. There is a typical lower resistance.
[0081]
Experimental example 3
As Comparative Examples 6, 7, 8, 9, and 10, the surface resistivity on the back surface was 10 ⁶ Ω / □, 10 ⁸ Ω / □, 10 ¹² Ω / □, 10 ¹⁴ Ω / □, 10 ¹⁶ The surface resistivity of each intermediate transfer belt 8 of Ω / □ and the back surface in the first embodiment is 10 ¹⁰ The image quality of an output image was compared and examined in an image forming apparatus using the Ω / □ intermediate transfer belt 8.
[0082]
Table 3 shows the results of a comparative study of the images obtained by the intermediate transfer belts 8 with respect to the surface resistivity on each back surface provided in Comparative Examples 6 to 10 and Example 1.
[0083]
[Table 3]

[0084]
The determination of the density unevenness level and the image dust level in the above table is performed by visual sensory evaluation. The criteria are as follows: ○ = no occurrence, Δ = level that occurrence is slight and does not cause a practical problem, x = occurrence is remarkable This is an unacceptable level in terms of image quality.
[0085]
In this study, the surface resistivity on the back surface was 10 ¹⁴ In the intermediate transfer belts 8 of Comparative Examples 9 and 10 having Ω / □ or more, it was confirmed that the above-described density unevenness occurred despite the installation of the charge removing brush 15.
[0086]
This density unevenness also increases the surface resistivity on the back surface of the intermediate transfer belt 8 and causes the separation discharge to occur in the smaller gap α, as in the case of the experimental example 2 above. This is considered to be caused by the weakening of the discharge suppressing effect.
[0087]
Further, the surface resistivity on the back surface is 10 ⁸ In the intermediate transfer belts of Comparative Examples 6 and 7 having Ω / □ or less, a phenomenon (hereinafter, referred to as “image dust”) in which line images and character images are blurred due to toner scattering in the primary transfer process is remarkable. It was confirmed that it became.
[0088]
The present inventor considers the mechanism by which the image dust depends on the surface resistivity of the intermediate transfer belt 8 as follows.
[0089]
It is considered that the cause of the occurrence of image dust is toner flying in a gap portion before the primary transfer portions Ta to Td. Therefore, when the surface resistivity on the back surface of the intermediate transfer belt 8 is low, the electric field applied to the gaps before and after the primary transfer portion becomes stronger than when the surface resistivity on the back surface is high, so that the primary transfer portions Ta to Td Starts to fly from the photosensitive drums 2a, 2b, 2c, and 2d onto the intermediate transfer belt 8 from the upstream side. For this reason, it is considered that the lower the surface resistivity on the back surface of the intermediate transfer belt 8, the worse the image dust.
[0090]
From the result of the comparative study of Experimental Example 3, the surface resistivity of the intermediate transfer belt 8 was 10 ⁸ Ω / □ or more 10 ¹⁴ By setting the resistance to Ω / □ or less, it is possible to maintain the effect of preventing the image unevenness by the charge removing brush 15 and also to prevent the occurrence of image dust.
[0091]
Considering the results of Experimental Examples 1 to 3, in the image forming apparatus having the intermediate transfer belt 8, the toner image primarily transferred to the intermediate transfer belt 8 is subjected to the secondary transfer before reaching the secondary transfer portion Tn2. In this embodiment, in the image forming apparatus which is charged by the pre-charging means 14 and the tribo value is made uniform and increased by the charge of the toner constituting the toner image, in this embodiment, the secondary transfer pre-charging means which is the driving roller 11 A neutralization brush 15 is provided immediately downstream of the charging member facing the charging means before the secondary transfer, which constitutes the counter electrode of No. 14, and the surface resistivity of at least the portion of the drive roller 11 which contacts or approaches the intermediate transfer belt 8 is set to 10 ¹⁰ Ω / □ or less, and the surface resistivity of the back surface of the intermediate transfer belt 8 on which the toner image is not formed is 10 ⁸ Ω / □ or more 10 ¹⁴ By specifying Ω / □ or less, it is possible to prevent the occurrence of image density unevenness due to charging before secondary transfer. The method of measuring the back surface resistivity of the intermediate transfer belt 8 and the surface resistivity of the drive roller 11 is measured by the method described in the first embodiment.
[0092]
Note that, in the present invention, the overall configuration of the image forming apparatus is not limited to that of the present embodiment. The surface resistivity of the member facing the charging unit before the secondary transfer is set to 10%. ¹⁰ Ω / □ or less, the surface resistivity on the back surface of the intermediate transfer member is 10 ⁸ Ω / □ or more 10 ¹⁴ The present invention can be applied to all types of image forming apparatuses defined in the range of Ω / □ or less.
[0093]
Example 2
The image forming apparatus according to the second embodiment of the present invention is the same as the full-color image forming apparatus such as the electrophotographic printer of the first embodiment shown in FIG. Therefore, description overlapping with the description in the first embodiment will be omitted.
[0094]
In the present embodiment, instead of the discharging brush 15 employed in the first embodiment, a discharging needle 15 ′ which is a needle-shaped member is used as the discharging means 15 for discharging the intermediate transfer belt 8 after the secondary transfer has been charged. ing.
[0095]
As shown in FIG. 5, the static elimination needle 15 ′ has a needle shape having a height of 3 mm, a pitch of 2 mm, and a tip edge R of 0.1 mm or less in the longitudinal direction of the SUS plate having a thickness of 0.1 mm and a length of 15.0 mm in the transverse direction. Of the mountain. The length in the longitudinal direction is the same as the length in the direction intersecting the moving direction of the intermediate transfer belt 8.
[0096]
The installation position is the same as that in the static elimination brush 15 of the first embodiment shown in FIG. 4, and the center of the tip of the static elimination needle 15 ′ is also located at a distance of 1.0 mm from P. It is electrically grounded.
[0097]
Even when the static elimination needle 15 ′ is used, the peeling discharge generated between the intermediate transfer belt 8 and the driving roller 11 can be effectively suppressed, and the image density unevenness due to the pre-secondary transfer charging can be reduced. Occurrence can be prevented.
[0098]
Since the static elimination needle 15 'is harder and more durable than the static elimination brush 15, it has advantages such as being free from deformation during a long-time continuous printing operation and having excellent durability.
[0099]
On the other hand, the static elimination brush 15 used in the first embodiment is one in which brushes having fine tip shapes are individually arranged at a high density, and thus are compared with the static elimination needle 15 ′ in the present embodiment. It has advantages such as high static elimination performance and stability.
[0100]
As described above, similarly to Embodiment 1, in the image forming apparatus having the intermediate transfer belt, the toner image primarily transferred to the intermediate transfer belt is charged by the secondary transfer pre-charging unit before reaching the secondary transfer unit. In an image forming apparatus in which the tribo value is made uniform and increased by applying a charge to the toner constituting the toner image, immediately downstream of the pre-secondary-transfer-charging-means facing member constituting the opposing electrode of the secondary-pre-charging means. Even when a static elimination needle is used in place of the static elimination brush as the static elimination member to be installed, the surface resistivity of at least the portion of the pre-secondary transfer charging unit facing member that contacts or approaches the intermediate transfer member is 10%. ¹⁰ Ω / □ or less, and the surface resistivity of the back surface with respect to the side of the intermediate transfer member on which the toner image is formed is 10 ⁸ Ω / □ or more 10 ¹⁴ By specifying Ω / □ or less, it is possible to prevent the occurrence of image density unevenness due to charging before secondary transfer.
[0101]
The charge removing member is not limited to the charge removing brush of the first embodiment or the charge removing needle of the second embodiment, and the configuration such as the size and material is appropriately determined.
[0102]
【The invention's effect】
As described above, the image forming apparatus of the present invention has an intermediate transfer body on which a developer image is primarily transferred from an image carrier, and transfers the developer image primarily transferred on the intermediate transfer body to a secondary transfer unit. And an image forming apparatus that obtains an image by secondary transfer to a transfer material at a position downstream of the primary transfer portion and upstream of the secondary transfer portion with respect to the moving direction of the intermediate transfer member. Charging means for charging the developer image to the same polarity as the charging polarity of the developer image, and charging means for charging before the secondary transfer with the intermediate transfer member interposed therebetween; And a member facing the charging means before the secondary transfer which is in contact with or close to the back surface of the developer carrying surface of the intermediate transfer member. The portion immediately downstream of the charging means facing member before the next transfer and on the back side of the developer image carrying surface of the intermediate transfer body. Is location, it has a charge removing means for neutralizing the intermediate transfer member, the surface resistivity of the contact portion between at least the intermediate transfer member of the secondary transfer charging unit facing member 10 ¹⁰ Ω / □ or less, and the surface resistivity of the back surface of the developer carrying surface of the intermediate transfer member is 10 ⁸ Ω / □ or more, 10 ¹⁴ Within the range of Ω / □ or less, the occurrence of peeling discharge between the intermediate transfer member and the member facing the charging means before the secondary transfer is suppressed, and the occurrence of density unevenness is prevented to provide a good image. The effect of being able to do is obtained.
[Brief description of the drawings]
FIG. 1 is a schematic configuration diagram illustrating an example of an image forming apparatus according to the present invention.
FIG. 2 is a cross-sectional view showing a driving roller as an example of a member for facing a charging unit before secondary transfer according to the present invention.
FIG. 3 is a plan view showing a charge removing brush which is an example of a charge removing member according to the present invention.
FIG. 4 is a schematic configuration diagram showing an arrangement of a charge removing member according to the present invention.
FIG. 5 is a plan view showing a static elimination needle as another example of the static elimination member according to the present invention.
[Explanation of symbols]
1Y, 1M, 1C, 1D image forming unit
2a, 2b, 2c, 2d Photosensitive drum (image carrier)
5a, 5b, 5c, 5d Primary transfer roller
8 Intermediate transfer belt (intermediate transfer body)
11 Driving roller (the member facing the charging means before secondary transfer)
12 Secondary transfer opposed roller (roller member)
13 Follower roller (roller member)
14 Charging means before secondary transfer
15 Static elimination roller (static elimination member)
15 'static elimination needle (static elimination member)
17 Source of charging bias before secondary transfer
19 Secondary transfer roller
Ta, Tb, Tc, Td Primary transfer unit
Tn2 Secondary transfer unit

Claims

An image carrier on which a developer image is formed, and an intermediate transfer body on which the developer image is primarily transferred from the image carrier at a primary transfer unit, and primary transfer is performed on the intermediate transfer body. An image forming apparatus for obtaining an image by secondary-transferring the developer image onto a transfer material at a secondary transfer unit,
Further, the intermediate transfer member is provided at a position downstream of the primary transfer portion and upstream of the secondary transfer portion with respect to a moving direction of the intermediate transfer member and on a side of the developer image bearing surface of the intermediate transfer member. A pre-secondary transfer charging unit that charges the developer image carried on the surface to the same polarity as the charging polarity of the developer image; and a position facing the pre-secondary transfer charging unit with the intermediate transfer member interposed therebetween. In the image forming apparatus, the charging device facing the secondary transfer pre-transfer member contacting or close to the back surface of the developer carrying surface of the intermediate transfer body,
Further, the intermediate transfer member is disposed immediately downstream of the member facing the charging means before the secondary transfer with respect to the moving direction of the intermediate transfer member and on the back side of the developer image bearing surface of the intermediate transfer member. Having a static elimination means for eliminating static electricity,
A surface resistivity of at least a contact portion of the pre-secondary transfer charging unit facing member with the intermediate transfer member is 10 ¹⁰ Ω / □ or less;
The surface resistivity of the back surface of the developer carrying surface of the intermediate transfer member is in the range of 10 ⁸ Ω / □ or more and 10 ¹⁴ Ω / □ or less.
An image forming apparatus comprising:

The immediately downstream portion of the pre-secondary transfer charging member facing member where the static elimination member is installed is a position downstream of the pre-secondary transfer charging member facing member in the moving direction of the intermediate transfer member, and 2. The image forming apparatus according to claim 1, wherein the image forming apparatus is located at a position facing the pre-transfer charging unit.

The image forming apparatus according to claim 1, wherein the intermediate transfer member has an endless belt shape movably stretched by a plurality of roller members.

4. The image forming apparatus according to claim 3, wherein the member facing the charging unit before the secondary transfer is any one of the plurality of roller members that stretch the intermediate transfer member.

The static elimination means is an electrically grounded brush-like member,
The image forming apparatus according to claim 1, wherein:

The static elimination means is an electrically grounded needle-like member,
The image forming apparatus according to claim 1, wherein: